The present invention relates generally to radiating transmission lines, particularly coaxial cables having helically disposed slots, and to radio communication systems that use such radiating transmission lines.
Radiating coaxial cable has been used for many years in various types of radio communication systems. An improved radiating cable is disclosed in commonly-owned U.S. Pat. No. 5,809,429, which is incorporated herein by reference in its entirety. An embodiment of this improved cable contains one row of slots in the cable""s outer conductor which are configured to produce a radiated field polarized perpendicularly to the axis of the cable to avoid the radiation of a field polarized parallel to the cable axis and to provide coupling energy between the interior of the cable and the slots. Another embodiment of this improved cable contains two parallel rows of slots disposed in the outer conductor diametrically opposite each other so that the cable performance would be independent of the wall-mounting position.
In practice, when using the cable with a single row of slots, attention must be given to the slot position during the mounting of this cable on a wall. Preferably, for best performance, all the slots should be facing outward away from the wall. A cable mounted with all of the slots facing outward away from the wall performs superior (see FIG. 1a) to a cable which has slots over a substantial length of cable facing inward towards the wall (see FIG. 1b). FIG. 1c illustrates a cable 10 containing a row of axially aligned slots 11 according to one embodiment of the cable disclosed in commonly-owned U.S. Pat. No. 5,809,429, incorporated by reference above.
Cable machines used in industry today tend to twist the cable as the cable is formed during manufacturing and/or reeled for shipping. The effect of the cable twist is the random rotation of slots over unpredictable lengths of cable. It has been observed that during cable manufacturing the slots of the cable can be rotated 360xc2x0 over 180 feet of the cable. For example, this rotation can occur abruptly for a substantial length of cable so that the slots switch from being rotated 0xc2x0 in the circumferential direction to being rotated 180xc2x0 over a length of cable, and then again being rotated another 180xc2x0 back to the first position for the next length of cable, where the rotations between 0xc2x0 and 180xc2x0 are random.
Another problem associated with the manufacture of radiating coaxial cable having all slots aligned in a row along the axis of the cable is mechanical slot compression. Such a cable is manufactured by wrapping the outer conductor, already having the slots formed therein, around the cable. During wrapping, the slots are compressed in the circumferential direction with respect to the cable causing the slots to become narrower. This mechanical slot compression results in less slot area through which the cable can emit or receive a signal. To remedy mechanical slot compression, tape is often affixed to the outer conductor before wrapping. The tape reinforces the outer conductor to help maintain the shape of the slots during wrapping. However, taping does not prevent slot compression; rather, it lessens its effect. Further, taping increases manufacturing time and expense.
FIGS. 1a and 1b provide an example which illustrates the effect that facing slots towards the wall has on the received signal level. A 180 foot length of cable that experienced the aforementioned twisting during manufacturing and reeling contained a 90 foot mid-portion having slots rotated so as to face inward towards the wall. The remaining portion of the cable was situated so that the slots faced outward away form the wall. This degree of slot rotation is not uncommon for a cable that has experienced twisting during manufacturing and reeling. The coupling amplitude of a 900 MHz signal was measured along the length of this cable. The type of signal obtained is shown in FIG. 1b and is undesirable because the drop in signal strength can result in degraded information received over such a long interval or a complete loss of communication over the interval. The magnitude of this null can be appreciated by comparing FIG. 1b with FIG. 1a. Thus, there is a need to remedy this effect in order to use a radiating cable in a radio communication system that is to provide a steady signal. Furthermore, there is a need for a radiating cable whose performance is independent of the wall mounting position of the cable.
An object of some embodiments of the present invention is to provide an improved radiating coaxial cable which can be mounted close to, or even on, a wall (even a metallic wall) or other surface independent of the cable orientation without significantly degrading the operation of the radio communication system in which the radiating cable is used.
Another object of some embodiments of the present invention is to provide an improved radiating coaxial cable which can be manufactured without experiencing mechanical slot compression.
In accordance with one embodiment of the present invention, the foregoing objectives are realized by providing a radiating coaxial cable having a longitudinal axis comprising an inner conductor having a longitudinal axis wherein the axis of the inner conductor defines the axis of the cable. The cable also comprises a dielectric material surrounding the inner conductor. A continuous outer conductor surrounds the dielectric and is in direct contact therewith and is spaced from the inner conductor. The outer conductor has a plurality of slots disposed therein with adjacent slots being spaced in the axial direction. According to some embodiments of the present invention, the slots are helically disposed in the circumferential direction.
According to some embodiments of the present invention, the radiating coaxial cable having helically disposed slots in the cable""s outer conductor can be mounted without regard to the direction which the slots are facing in relation to the signal transmitter or receiver.
Also according to some embodiments of the present invention, an improved radio communication system is provided which includes the above radiating cable located within or adjacent to a prescribed area containing a multiplicity of radio transmitters, receivers or transceivers (xe2x80x9cradio unitsxe2x80x9d), which may be either mobile or fixed. Signals are transmitted to and received from the various radio units via the radiating cable.